Researchers Propose a Renewable Energy Cycle Based on Co-electrolysis of Water and CO2 to Produce Syngas
|Schematic illustration of a generic liquid-fuel energy cycle utilizing a renewable electrical source. Credit: ACS. Click to enlarge.|
Researchers at Northwestern University are proposing, and have begun experimental validation of, a renewable liquid-fuel energy storage cycle based on the co-electrolysis of H2O and CO2 using a solid oxide electrolysis cell (SOEC) powered by renewable electricity to produce syngas. The syngas is then in turn converted into liquid fuels (e.g., methanol or synthetic hydrocarbons) which could be used in a direct fuel cell.
The direct fuel cell produces electricity, with water and CO2 as byproducts of the oxidation of the liquid fuel in the fuel cell. These would be captured and recycled back into the co-electrolysis process.
The roundtrip efficiency achievable for either a methanol or a liquid hydrocarbon cycle should be greater than that of a hydrogen cycle for storing energy from renewable sources, the researchers calculated. A paper on their work was published online 19 May in the ACS journal Energy & Fuels.
Most of the major steps of the proposed liquid fuel cycle—catalytic fuel production from syngas; storage and transport of the fuel; and operation of fuel cells on methanol or liquid hydrocarbons are either already in widespread use or have been demonstrated. The only step not yet extensively investigated is the electrolytic production of syngas, the primary topic of the paper.
SOEC technology is based on solid oxide fuel cell (SOFC) technology, and utilizes similar designs: Ni-YSZ fuel electrodes, YSZ electrolytes, and LSM-YSZ air electrodes. Although there are “open issues” with longevity, the SOEC devices have “good performance” in splitting water, the authors note. SOECs also offer chemical flexibility, and are able also to produce CO from CO2. Hydrogen and CO are the basic components of syngas.
The team carried out electrolysis was carried out at 700-800 °C using solid oxide electrochemical cells with H2O-CO2-H2 mixtures at the Ni-YSZ cathode and air at the LSCF-GDC anode. (YSZ = 8 mol%, Y2O3-stabilized ZrO2; GDC = Ce0.9Gd0.1O1.95; and LSCF = La0.6Sr0.4Co0.2Fe0.8O3).
The cell electrolysis performance decreased only slightly for H2O-CO2 mixtures compared to H2O electrolysis and was much better than for pure CO2 electrolysis. Mass spectrometer measurements showed increasing consumption of H2O and CO2 and production of H2 and CO with increasing electrolysis current density.
Electrolyzers operated on 25% H2, 25% CO2, and 50% H2O at 800 °C and 1.3 V yielded a syngas production rate of 7 sccm/cm2.
The CO2-H2O electrolysis process is able to use the heat from the exothermic liquid-fuel production process can be used to help boil liquid water and preheat the gases entering the electrolyzer, thereby increasing the efficiency of the overall process.
This proposed energy cycle is CO2 neutral but has the significant advantage that the liquid fuels are very similar to current fossil fuels; the technological and infrastructure barriers to introducing this energy cycle are thus significantly less than for a hydrogen energy cycle, allowing a easier transition to a carbon-neutral and cost-competitive liquid fuel economy using renewable electricity. Furthermore, thermal synergies in the liquid fuel cell should allow a substantially higher round-trip energy efficiency than a hydrogen cycle.—Zhan et al. (2009)
Zhongliang Zhan, Worawarit Kobsiriphat, James R. Wilson, Manoj Pillai, Ilwon Kim and Scott A. Barnett (2009) Syngas Production By Coelectrolysis of CO2/H2O: The Basis for a Renewable Energy Cycle. Energy Fuels, Article ASAP doi: 10.1021/ef900111f